class Father{
    constructor(a,b){
        this.a = a 
        this.b = b 
    }
    add(){
        console.log('add'+this.a + this.b)
    }
    multiple(){

    }
}
class Son extends Father{
    constructor(a,b){
        super(a,b)
    }
    add(){
        console.log(this.a + this.b)
    }
    multiple(){
        console.log('multiple'+this.a + this.b)
    }
}
let a = new Son(10,1)
a.multiple()